The present invention relates to processes and systems for effecting phase separation of solids and liquid in metal hydroxide sludge and, more particularly, to novel and improved methods of metal hydroxide sludge dewatering involving the freezing and thawing of the sludge.
Metal salts such as aluminum sulfate (alum) have been used in treatment processes since the early 1900's for the removal of color and turbidity from raw water sources. These hydroloyzing metals react with the natural alkalinity in the raw water and form light fluffy floc particles. This floc includes the metal hydroxide particulate, raw water solids and a significant "water of hydration". Prior to the 1960's, alum sludges were most commonly returned to the stream or lake. With the advent of the Federal Pollutant Discharge Regulations, this practice is no longer allowed.
The hydroxide sludges are very difficult to dewater by ordinary means because of the water molecules that are bound in the sludge. The solids are not easily removed by gravity consolidation and draining. These gelatinous sludges will not exceed 6% to 8% solids (by dry weight) through natural gravity thickening in lagoons. A 20% solids concentration is generally considered the minimum for handling by earth moving equipment and landfilling. Alternative methods of dewatering the sludge are required to meet these conditions.
A common method of disposal of sludges became the discharging of the water plant waste to a sewage treatment system. The complexity of dewatering and the associated costs are then transferred to the sewer agency and are often reflected in associated discharge fees. When discharge to the sewers is not feasible, mechanical methods such as filter presses and centrifuges can be utilized to separate the solids from the water. Since the sludge does not dewater easily, it requires pretreatment with ploymers, line, and often fly ash. These materials add solid mass to the sludge while helping the sludge to release its bound water. Quite often the thickened sludge has to be treated at a pH above 10 standard units. This high pH can present problems in suitable disposal for both the sludge cake and filtrate. The chemical control is important in these mechanical dewatering processes. The chemical facilities not only add capital costs to sludge handling, but also increase the operation and maintenance costs. In addition, the nature of the sludge greatly affects the final solids concentration and characteristics of the dewatered solids.
Unlike the mechanical dewatering alternatives (which dewater alum sludge by adding solids and squeezing water out), freezing and thawing produces a permanent physical/chemical change in sludge characteristics. When the sludge undergoes complete freezing, the water crystalizes and breaks the bond of hydration. Upon thawing, the solid materials will separate and settle quite efficiently, with a water content low enough to permit handling of the solids by conventional techniques. The thawed sludge exhibits a more granular texture and will not return to its original gelatinous state when water is added.
It is known that freezing can change sludge of 2% to 6% solid concentration to 20% to 25% solids immediately following thawing. Since the solid material exhibits characteristics similar to fine sand, it will readily air dry and drain to concentrations greater than 40% solids (by weight). The resulting solids can be easily handled by earth moving equipment and are suitable for a landfill operation. A number of U.S. patents relate to methods and apparatus for separating solids from water by treatment involving freezing and thawing. For example, U.S. Pat. No. 3,713,303 discloses and pair of heat exchangers comprising tubes within casings and means for blowing air through the tubes at the end of each freezing cycle. U.S. Pat. No. 3,783,632 relates to apparatus for concentration of aqueous coffee extract including a tubular ice crystallizer and basket centrifuge ice separator.
Process patents in this field include U.S. Pat. No. 2,703,782, which describes the dewatering of sewage sludge by addition thereto of a coagulating agent, e.g., aluminum sulfate, then freezing, thawing and decanting, in a natural freezing-thawing manner of treatment of alum sludges. U.S. Pat. No. 3,098,733 relates to a refrigeration process for recovering fresh water from salt water which involves freezing and melting. Likewise, U.S. Pat. No. 3,253,419 discloses a refrigeration process for separation of components from a liquid mixture by fractional crystalization.
U.S. Pat. No. 3,817,048 discusses various prior apparatus and methods used in sludge treatment by freezing, thawing and decanting, and is directed to a process involving freezing the sludge in droplet form by contacting it with an inert and immiscible liquid, thawing the frozen droplets and then reducing the liquid content. U.S. Pat. No. 4,453,959 is concerned with a process for separating and purifying a crystalized component from a two-phase slurry of the crystalized component and liquified multi-component mixture by separation of the crystalized component in a purification process.
It is a principal object of the present invention to provide a process for removal of solids from water sludges by freezing, thawing and settling which is efficient and economical in relatively large-scale operations.
A further object is to provide a method for dewatering metal hydroxide sludge which makes maximum use of commercially available refrigeration equipment in a continuous-type process of freezing, thawing and effecting phase separation of water and solids.
Another object is to provide an efficient process for separating water and solids in metal hydroxide sludges utilizing a standard refrigeration circuit both to freeze and to assist in thawing of the sludge.
Other objects will in part be obvious and will in part appear hereinafter.